Once your mayo is emulsified, you cannot add any other ingredients, beit solid or liquid, and use a food processor or blender to incorporate it. You will overly agitate the properties that are holding the emulsification together and it will break. I seriously doubt that it was the temperature of the dijon, although it MIGHT be a very slight possibility. I've made tons of mayo and aioli and I have added both room temperature and cold ingredients to the finished product, beit gherkins, capers, diced veg., sriracha, ponzu, etc. The key is to fold it in using a spoon or spatula.
Remember that key point here was that the mayo had already reached the desired consistency BEFORE it lost it's emulsification. If it never emulsified and stayed runny, then it would most likely be due to the ingredients, temperature, and/or technique.
"Boldness has genius, power, and magic in it."
Johann Wolfgang Von Goethe